CD8+ memory T cells are one of the cornerstones of successful vaccination, and much is known about their generation, i.e. the pathways that determine nave CD8+ T cell activation, expansion & contraction, and the factors that affect the establishment of CD8+ memory T cell subsets. In contrast, we know relatively little about CD8+ memory T cell recall responses, upon which vaccine efficacy relies. Memory cells deal with individual virus-infected cells in vivo and, to do so effectively, they must respond to secondary infection (i.e., begin their recall response) by imposing their effector functions upon an infected cell before virus progeny has been released; for many virus infections, this means that - if they are to be maximally protective - memory T cells must act within hours of infection. Hence, my lab recently has begun to investigate the very early (d24 hours p.i.) recall responses of CD8+ memory T cells; and, so far as is possible, we measure the responses in vivo. Unpublished data (presented herein) show that: (i) CD8+ memory T cells initiate effector responses to virus infection in vivo within 3-6 hours (long before a single round of virus replication has been completed); and (ii) very surprisingly, in vivo cytokine synthesis (IFN, TNF & IL-2) is largely terminated soon thereafter (by 24 hours p.i.). This termination of effector function occurs despite the presence of immunostimulatory viral antigen, suggesting that there is active down-regulation of effector function by CD8+ memory T cells that have responded to infection. These events occur before the memory T cells have multiplied. These early recall responses, and the molecular mechanisms that control them, will be explored in four Specific Aims. 1. To investigate the in vivo regulation of CD8+ memory T cell early recall responses. Our observations regarding the rapid on/off expression of effector functions by CD8+ memory T cells will be expanded. 2. To identify and manipulate the molecular pathway(s) underpinning the early recall response of CD8+ memory T cells. The molecular mechanisms regulating the initiation, and the termination, of effector function(s) will be identified using several approaches. 3. To evaluate the role of dendritic cells in regulating these very early memory T cell responses. The majority of studies of DC / T cell interactions have (understandably) focused on priming of nave T cells. Here, we shall evaluate the role of DCs in the rapid in vivo responses of CD8+ memory T cells. 4. To generate and test a mouse line that would allow us, for the first time, to evaluate the importance of various proteins in regulating the recall responses of CD8+ memory T cells. We shall generate a transgenic mouse line which will allow the investigator to deletion a gene from CD8+ T cells at the time of his/her choosing. An infected mouse would mount a completely normal primary T cell response, and would establish a normal memory cell pool; only then would we delete the gene of interest.